In galvanizing operations, the metal usually steel, to be coated with zinc must be first cleaned to remove rust, dirt, scale, and other contaminating materials. Such cleaning permits the zinc coating to adhere successfully to the steel surface. In such galvanizing plants, it is conventional to use a vessel charged with acid to clean many forms of steel. Normally, the acid has a pH of about 1 and, typically, a strong acid is used; the most common acid beig sulphuric acid, though hydrochloric acid can be used. The vessel or container is heated to something less than boiling, since elevation of the temperature increases the chemical activity occurring in the tank and permits more rapid cleaning.
Sometimes, the galvanizing process will not be carried on perfectly and, as a consequence, the partially galvanized steel must be cleaned. Thus, it is necessary to return the incorrectly galvanized steel to the cleaning tank where it will be stripped of the zinc coating thereon.
After some period of time, the cleaning vessel contains a substantial amount of dirt, acidic contaminants, and various quantities of metal ions. The metal ions typically include iron and traces of metal elements such as manganese, copper, chromium, lead, tin, molybdenum, titanium, nickel, strontium, and zinc. Other trace elements may also be found in the acid bath after the cleaning of metals. This reduces the effectiveness of the acid bath. When the acid bath becomes substantially contaminated, it is no longer able to properly clean the steel. It has been, heretofore, necessary to dump all of such contaminated acid from the cleaning tank and replace it with a fresh batch of acid. However, in recent years with concern about the environment rising, it has become exceedingly difficult to dispose of large volumes, conventionally, of spent acid. While the acid may not be usuable commercially for cleaning the steel at this point, it remains a strong acid and most local laws forbid the dumping of a strong acid laden with metal ions into the sewer system. One alternative has been to cart the spent or contaminated acid away to an authorized disposal facility where it might be combined with available waste bases for neutralization of the acids. As can be appreciated, such disposal techniques are quite expensive.
To avoid such costly dumping, purification of the spent liquor has been attempted. One method contemplates placing an anode and a cathode in the pickle liquor and passing a D.C. current through the terminals and the bath. Elemental metals can be recovered in the vicinity of the cathode by the use of an electromagnet at or above the surface of the pickle liquor. These materials attach to the cathode momentarily, but do not plate thereon as that term is ordinarily understood in the art. The current flow through the pickle liquor changes the valence of the metal ions at the anode enabling the formation of insoluble salts or oxides which can be recovered from the pickle liquor by filtration. In this regard, see U.S. Pat. No. 3,801,481, issued Apr. 2, 1974 to the present inventor.
Another method is disclosed in U.S. Patent Application Ser. No. 732,479 filed Oct. 14, 1976 in which a tank is used which is divided preferably down the center by a permeable membrane. The membrane has pores which permits metal ions to pass therethrough in one direction and acid cations in the opposite direction, but does not permit passage of water. An anode is placed on one side of the membrane and a cathode is placed on the other side. A D.C. current source is connected to the terminals and the current flow through the bath is established. Elemental metals collect and adhere to the cathode. Acid is concentrated and recovered at the anode. Thus the impure pickle liquor is continually pumped to the vicinity of the cathode and purified acid is recovered from the anode side. This technique recovers far more than half of the metal at the cathode. The remaining acidic liquid in the vicinity of the cathode is transferred to a precipitating tank where zinc oxide waste known as "skimmings" can be introduced to raise the pH. A precipitating technique can then be used to recover all the remaining metal ions.